Conversion of hydrocarbons



July 22, 1958 F. H. BLANDING ET AL 2,844,518

' CONVERSION OF HYDROCARBONS Filed March 16, 1954 FORREST H. sunomcINVENTORS BY ATTORNEY WALTER J. SCHMIDT United States Patent CONVERSIONOF HYDROCARBONS Forrest H. Blanding and Walter J. Schmidt, Cranford,

Application March 16, 1954, Serial N 0. 416,462

5 Claims. (Cl. 196-50) This invention relates to the catalytic crackingof hydrocarbons to produce lower boiling hydrocarbons such as motorfuels.

.During the catalytic cracking of hydrocarbons, cycle oil is separatedfrom the reaction products and it has been known to recycle part or allof this cycle stock to the catalytic cracking reactor. Recycle crackingon some units has given very poor results, resulting in lower gasolineand higher carbon or coke yields than are obtained by once-throughcracking. It has now been learned that by a particular choice ofoperating conditions and recycle stock, extremely advantageous resultscan be obtained for recycle cracking.

One of the requirements of this invention is that a hydrocarbon oilwhich is to be recycled is a material within the boiling range of thefeedstock and more specifically that only material is recycled whichb'oils between the and 80% distillation temperatures of the feedstock.Material boiling above the final boiling point of the feedstock and infact material boiling much above the 80% distillation temperature of thefeed will be-a highly aromatic material which produces many times morecarbon or coke as does material boiling within the smaller range of1080% of the feedstock. Also material boiling below the initial boilingpoint of the feed and most material boiling lower than the 10% point ofthe feed is also quite aromatic and refractory.

Another essential feature of this invention is that the operatingconditions be selected to be a cracking severity suflicient to produceno greater than 40-50% by weight conversion, preferably 40 to 49% byweight conversion, to coke and to gasoline and lighter products based onthe fresh oil feedstock assuming no recycle was used. It has been foundthat catalytic craokin'gproduces high relatively uniform selectivity togasoline when cracking to conversion levels of about 40% or less byweight. When proceeding to :higher conversion :levels and particularlyto those above 50% by weight, unde'sirable :re-cracking of the gasolineto carbon or coke and 'gas takes place. By keeping the conversion levelon the'fresh feedstock portion of the-total feed .to lower thanabout 50%by weight and, preferably about-40% to 49% by weight, this recracking ofgasoline can be avoided.

The cycle stock above referred to is carefully fractionated to recover afraction corresponding "to '10-"80% of the distillation range of thefresh oil feed-or boiling within this range and theresulting fraction is"the one recycled in the present process. Selecting such a fraction, itis easily possible in the presentinvention byproper control of operatingconditions to recycle all :of this material to extinction.

It has been found that the proper severity of operation canautomatically be found by the us'e of equations giving the amount ofrecycle oil as volume percent of fresh oil feed and taking intoconsideration 'the'weight percent of carbon in the aromatic-ringhydrocarbons of'the fresh feed and the volume percent of thefreshfoilfeed boiling in the range of the fraction which is beingrecycled.

According to this invention conditions are given for producing thehighest amount of gasoline per unit of coke produced when recyclingrelatively large amounts of a selected recycle stock. Most catalyticcracking plants are limited in their ability to burn coke or carbon fromthe catalyst and with the present invention more gasoline per unit ofcoke is produced and even though a larger amount of gasoline is producedthere is no additional amount of coke or carbon laid down and hencethere is no additional burden on the regenerator. This is in. directcontrast to recycling of a bottom fraction which produces a lower ratioof gasoline to coke than the once-through operation.

The essence of the invention is the outstanding improvement obtained andonly obtained when all three basic requirements are met. First, theboiling range of the recycle oil must be in the correct range. Second,the severity of the operation must be carefully selected not toovercrack the oil feed stock on the first cracking pass. Third, therecycle rate for cracking to extinction must be in the correct range,dependent on the feed stock, and the recycle stock boiling'range.Provided the first requirement is met, the formulas to be givenhereinafter for describing the recycle rates will automatically fix thesecond important requirement and this is an important feature or thisinvention.

To describe the latter important feature further, ithas been found thatthere is a very definite relationship between the severity of thecracking operation and the amount of recycle oil required to crack toextinction all material within a fixed boiling range, In order toincrease recy-cle, once all cycle stock in a given boiling range isbeing recycled, it is necessary to decrease the cracking severity. Bythus decreasing the cracking severity per pass, it is possible toachieve higher recycle rateswithont increasing the boiling range of thematerial recycled.

However, it is the important objective of this invention to achieve theproper recycle rate. If severity is maintained too low, say in the orderof 25-30% equivalent conversion in a once-operation, excessive-recyclewill be required. This will be costly, and have little benefit inproduct value. If recycle is too low, cracking severity will in turn betoo high and this will result in overcracking and poor productdistribution. Maintaining the recycle rate .in

accord with the formulas will thus fix the cracking severity in theproper range.

In the drawing there is shown one form of apparatus adapted to carry outthe present invention.

Referring now to the drawing, the reference character 10diagrammatically designates a reactor for carrying out catalyticcracking of hydrocarbons such as gas oil, .certain crude oils and otherrelatively high boiling hydrocarbon stocks, preferably boiling between600 and L000- 1l-00 F. The oil, preferably in a preheated condition, isintroduced into the reactor through line 12. Hot regenerated catalyst ata temperature of about 85.0-1l50 F. is introduced into line 12 to line14. In a fluid catalyst operation the catalyst to oil ratio fed to thereactor 10 may vary between about 5 to 20. The temperature in .thereactor 10 may be between about 850,and 10.00" F. In a fluid process theweight of oil per ,hour .per weight of catalyst (which is expressed asw./hr./w.) in the re-.

actor is usually between about 1 and 10, but may be higher. The catalystis preferably silica alumina .catalyst but other cracking catalysts suchas silica-magnesia, .acid treated bentonitic clays etc. may be used. Infiuidoperations the catalyst is finely divided with the majority of theparticles being between about 20 and ,microns.

Ground or microspherical catalyst particles of different sizeranges'may'be used. The present invention ,may also be used with fixedbed or moving bed processes.

-Provisi'on=is also madefor recyclinga selected fraction to the reactorIOthroug'h-line 16. It is to be understood that showing of theintroduction of oil and catalyst is diagrammatic and other points ofintroduction of these materials may be used. The amount and selection ofthe recycle oil passing through line 16 will be hereinafter described ingreater detail. Spent catalyst from the reactor is passed through line18 to regenerator 22 wherein the temperature is maintained between about1000 F. and 1200 F. to burn off carbonaceous deposits or coke from thecatalyst and the regenerated catalyst is then returned to the reactorthrough line 14.

The. cracked reaction vapors pass overhead from the reactor 10 throughline 24 and pass to the lower portion of fractionating tower 26 whereinthe reaction products are fractionated to recover desired products.

The vaporous reaction products are fractionated in the fractionator 26to separate a bottoms fraction which is withdrawn through line 28 andmay be discarded from the system via line 30 or may be all or in partpassed through line 32 to a second fractionating tower 34. A cycle oilfraction is withdrawn further up the fractionating tower 26 through line36 and passed to second fractionating tower 34 for further fractionationas will presently be described in greater detail.

A heating oil fraction is preferably withdrawn further up through thetower 26 from trap out tray 38 and through line 42. The overhead vaporsare passed through line 44 and through condenser 46 and then throughgas-liquid I separator 48 for separating liquids from gases. The gas iswithdrawn overhead through line 52 and the liquid is withdrawn from thebottomvof the separator 48 through line 54 and from the system throughline 56 as a gasoline cut. A portion of the liquid may be recycled asreflux to the top of the fractionator 26 through line 58 by pump 62.

Returning now to second fractionating tower 34, the cycle oil to berecycled is a carefully selected fraction boiling between about 650 and9001000 F. depending on the boiling range of the fresh oil feed. Thecycle oil introduced into second fractionating tower 34 through lines 32and 36 is further fractionated to separate a bottoms fraction containingthe highest boiling constituents. The bottoms fraction is withdrawn fromthe system through line 66 and may be used as fuel oil or for any otherdesired purpose. During fractionation in tower 34 the highest and lowestboiling constituents are separated and the cycle oil to be recycled iswithdrawn from trap out tray 67 at an intermediate portion offractionator 34 through line 68 and recycled to reactor 10 through line16. A portion of this recycle oil may be withdrawn from the systemthrough line 70, if desired. Lighter vapors from fractionator 34 passoverhead through line 72 to main fractionating tower 26.

Where the fresh oil feed has a boiling range of about 600 to 1050 F.,the boiling range of the selected cycle oil stock to be recycled isabout 675900 F. Where the fresh oil feed has a boiling range of about800 to 1100 F., the boiling range of the cycle oil stock to be recycledis about 930 to 1020 F. The cycle oil to be recycled is selected to havea boiling range between the 10% and 80% boiling points of the fresh oilfeed or virgin charging stock.

As above pointed out the present invention provides a method ofproducing the highest amount of gasoline per unit of coke produced.According to the present invention it is desirable to maintain totalconversion or conversion to coke, gasoline and lighter products betweenabout 40% and 50% by weight and preferably below 49% by weight on thefresh oil feed, assuming no recycle was used, or in other words on aonce-through basis. Under these conditions the amount of cycle oilrecycled may amount to between about 40% and 250% by volume on the freshoil feed. Under these conditions superior production distribution isobtained in heating oils and naphtha yields, and cycle oil stock qualityis improved.

The low reactor severity to get low conversion is obtained by having alow reactor catalyst holdup in fluid catalyst operations.

It has been found that there is relationship between the weight percentof carbon in the aromatic ring hydrocarbons in the fresh feed and theamount of cycle oil which should be recycled in order to convert all ofthe cycle oil into more valuable cracked products, while maintainingsuperior yields. The following equations give the amount of recycle oilas volume percent of fresh feed, the first equation giving the minimumrecycle and the second equation giving the maximum recycle, forrecycling cycle stock to extinction.

The percent by volume of recycle oil (minimum) as volume percent offresh feed where A is the weight percent of the total carbon which ispresent in aromatic ring hydrocarbons in the fresh feed and B is thevolume fraction of the fresh oil feed boiling in the range which isbeing recycled.

Recycled oil as volume percent on fresh oil feed (maximum) thenomenclature being the same in the two equations. The value of A mayvary between about 15 and 30'and the value of B may vary between about0.3 and 0.7.

As one example for comparing the present invention with a once-throughoperation the following results are given. A once-through operation gaveabout gasoline for 5 weight percent of coke so that the ratio ofgasoline per unit of coke was 10. A recycle operation conducted inaccordance with the above principles produced 57.7% gasoline and only3.8% coke giving a gasoline to coke ratio of 15. This is an increase ofover 50% in the amount of gasoline produced per unit of carbon produced.As most catalytic cracking plants are limited in their ability to burncoke formed on the catalyst this is a most important advance in thecatalytic cracking art. In contrast to the results obtained by recyclingin the present invention, it is noted that the conventional type ofrecycle of a bottom fraction boiling above about 800 F. produced a lowergasoline to coke ratio than a oncethrough operation.

The weight percent of carbon in the aromatic ring hydrocarbon compoundsin the fresh feed is obtained by a method which is a modification of Dr.Fenskes method reported in the Journal of Institute of Petroleum, volume36, No. 322, October, 1950. Fenske separates the aromatic containingmolecules from other hydrocarbons using silica gel and then obtains thepercentage of carbon atoms actually in aromatic ring structure in thearomatic fraction, based on measurements of refractive index and othersimple physical measurements of this fraction. While the Fenske methodis not entirely precise for analyzing aromatics in petroleum, the Fenskevalues are closely related to values obtained by more precise butcomplex and costly methods and thus are useful for the purpose of thisinvention.

Using the equations above given, the minimum and maximum amounts ofrecycle will be given for example of a cycle oil stock separated fromcatalytically cracked products and having a boiling range of about 650to 900 F. The gas on fresh feed is a West Texas Gas oil of 22% by weightof carbon in the aromatic hydrocarbon rings which has boiling betweenthe 650 and 900 F. temperature levels. Aromatic rings in this exampleand in all succeeding examples herein disclosed are determined by thepreviously described Fenske method. Thus A is 22 weight percent andB is0.7. Using these values in the equations it will be found that theminimum amount of recycle which may be used is about 70% on the freshoil feed and 128% recycle on the fresh. oil feed as the maximum. Anyvalue between these two limits may be used.

For different feed stocks the minimum recycle rates may vary from about50 to 72%.

As a specific example a mixed sweet gas oil of Gulf coast origin havingthe following properties was used:

24.3 API gravity:

Initial boiling point 600 F- at 675 F. at 722 F. at 965" F. at 1000" F.Final boiling point 1050 F. Weight percent of carbon in the aromaticring compounds of this feed equals 18 In the specific example theboiling range of the cycle oilstock from a. catalytic cracking operationwas about 675-900 F. Thefollowing data compare once-through 20 operation(no recycle) with ordinary. recycle operation and with the process ascarried out following the teachings of the present invention.

quires a. favorably low amount. of catalyst to effect the the resultsobtained with the process of thepresent inven-.

tion. In operation 5 the boiling range of the recycle is specific, butthe recycle rate is too low, being 23% as compared with the 54% minimumamount for this feed stock required by the invention. Results show onlya very small gain in gasoline ratio of from 10.8 to 11.5. This compareswith a gain of from 10.8 to 22 when using the process of the presentinvention. Operation 6 shows" that merely operating at recycle rates inthe same range as that of the invention will not give improved results.In operation 6 the entire cyclestock boiling within the range of thefeed is recycled at the same recycle rate as in operation 4. Yetgasoline to coke ratio is only 9.8, an order of magnitude lower than the22 of operation. 4.

Table 1 Operation l. 2 3 4 5 6 7 Type Ordinary Recycle This In- RecycleRecycle This Invention ventlon Vol. percent Recycle on Fresh Feed. 5 3373. 5 23. 6 74.0 55 Boiling Range of Recycle, F 844-1, 650-1, 100675-900 675-900 650-1, 100 675-900 Yields of Products: I

Gas, wt. percent 8. 7 9.0 11.1 7. 6 10. 9 10.1 7.8 Vol. percent MotorGasoline, O4-430 F. on

Fresh feed 53. 4 .51. 7 55. 2 62. 1 68. 8 54.8 59. 6 Vol. percentHeating Oil (430-650 F.), on

Fresh feed 16. 9 17.0 19. 7 22. 2 18. 5 19. 7 21. 6 Coke, wt. percent onfresh feed 4. 9 4.9 7.1 2. 8 5. 1 5. 6 2. 9 Ratio, Gasoline/Coke 10.010.6 7.8 22 11.5 9.8 20.7 a Total Conversion by wt. to coke, gasolineand lighter products 60 59 66 64 67 03 63 Total Conversion by weight tocoke, gasoline and lighter products calculated on the fresh feedcomponent on a once-through basis 60 56 53 45 59 44 47 OperatingConditions:

Temperatur 930 930 930 930 930 930 930 Catalyst/Oil ratio by wt. 10 1010 10 10. 2 10. 1 11. 1 rJW 1- 3 2.0 1. 9 12.0 1. 94 12.0 11. 5

In the above table it will be noted that operations 2 and 3 are ordinaryrecycle operations known in the industry and both include bottomsfraction recycle. In operation No. 2 the conversion is lower whenconducted to the same coke yield as is also the gasoline for the samecoke. Adding more recycle oil raises the coke further giving a littlemore gasoline due to cracking of cycle stock. Also higher conversion isobtained. However, the ratio of gasoline to coke is no higher for therecycle operation in operations 2 and 3. Comparing operations 1 and 4show 4 to give an outstanding advantage.

In operation No. 4 the recycle oil fraction was carefully selected andfractionated to have a boiling range between 675900 F. (within 10-80%boiling range of fresh feed).

The percent by volume of the fresh feed boiling in the range of thecycle oil which is to be recycled is about-- 60%, so that B is 0.6. Theformula calculates about 54% recycle as the lower limit and 100 as theupper limit. The actual recycle used in operation No. 4 was 73 which isabout in the middle of the range. The gasoline to coke ratio was morethan double of any of the operations 1, 2 and 3.

it will be noted that there is a drastic change in reactor severity orcatalyst holdup in the operation No. 4 as compared to the otheroperations. The reactor catalyst holdup corresponding to w./hr./w. is 12in operation 4 (low reactor severity) comparedto not more than 2 inoperations 1, 2 and 3. Because higher w./hr./w. values mean lesscatalyst in the reactor, operation4 also re- The extremely criticalnature of the selection of the cycle stock is emphasized by operations 4and 6. Also, it should be noted that heating oil yield is higher inoperation 4 than in operation 5' or 6, an additional advantage foroperation 4. Operation 6also shows that merely the specification ofrecycle of material in the boiling range of.

the feed is not adequate to gain the advantage. Excese sively highrecycle ratios offer no further advantage and are costly. The: minimumrecycle rate is exceedingly important. Operation 5 shows the loss foroperating at 23.6% recycle which is 30% below the lower limit to be veryserious. Yet operation 7, at 55% recycle, just within the specification,shows a gasoline ratio nearly as high as operation 4.

The important point of theinvention is the surprisingly highergasoline-coke ratios obtained when operated in critical accord with theinvention, as compared with values of only about half as great whendeviating from the teachings of the invention only to an. extent whichmight ordinarily be considered minor.

Because aromatic hydrocarbon compounds are difficult to crack, somewhathigher recycle ratios are required todefine the critical condition formore aromatic-feed stock. Virgin gas oils usually range between 15% and30 wt. percent carbon inthe aromatic ring hydrocarbons by the Fenskemethod, thus. the minimum recycle for such extremes mightvary from51-72%. i

The invention is not to be limited to merely the recycle of fractions inthe. range of 675-900F'F. as in the foregoing examples or operations.The proper fraction to select to obtain these excellent recycle resultsvaries with the feed stocks. This is further illustrated by thefollowing data. A feed stockboiling from about 800 to 1100 F. wascatalytically cracked on a once-through basis. The cycle stock from thisoperation was then fractionated into narrow boiling fractions and thesefractions were catalytically cracked to examine the quality of thematerial for further cracking as would be done in a recycle operation.The results when cracking all stock at the same Fraction Low, boilingbetween 730 and 830 F., gave a gasoline/coke ratio of only 7.4. Thisfraction boils within the same range shown to be very favorable in thepreceding Examples 4 and 7. However, it is not in accord with theprinciples of the invention, as in this case fraction Low boils belowthe of the feed, even though it does boil partly in the same range asthefee/d. Fraction Medium boiling within the 10-80% temperatures of thefeed gives an outstandingly better result of 12.3 gasoline/ coke ratio.Fraction High boiling above the 80% of the feed, gives a gasoline/ cokeratio of only 2.9.

The above results represent only once through cracking results on thecycle stocks. In a recycle operation the fractions are cracked once,then again, and again, until they are finally destroyed. Thus, in arecycle operation, the differences shown in cracking the above fractionsLow, Medium and High would be accentuated even more. The above exampleemphasizes the critical nature of the factors affecting the quality ofthe cycle stock for further cracking. In a recycle operation, operatedin accordance with the previous formulas, the recycle of stock Mediumwould produce a gasoline/coke ratio of about 22. Fraction High, on theother hand, would produce a gasoline/ coke ratio of less than 5. Thisshows strikingly how important it is to recycle the proper boiling rangematerial.

What is claimed is:

1. In a catalytic cracking process wherein virgin hydrocarbon oil havinga boiling range of about 6001050 F. is cracked in the presence of acatalyst and the vaporous cracked products are continuously fractionatedinto gasoline, heating oil, cycle oil and bottoms, and cycle oil is tobe continuously recycled to the cracking step and the selected cycle oilfraction is cracked to extinction, the

improvement which comprises carrying out the cracking step at arelatively low total conversion to coke, gasoline and lighter productsbelow about 49% by weight calculated on a once through basis,continuously fractionating the cracked products to recover a cycle oilfraction boiling between about 675 and 900 F., recycling at least 50% byvolume of said selected cycle oil fraction on the virgin oil feed to thecracking step, the amount of recycle oil being related to the weight percent of total carbon in the rings of the aromatic ring hydrocarboncompound content of the virgin oil feed and also to the amount of virginoil feed corresponding in boiling range to that of said selected recycleoil fraction, the amount of recycle oil for a selected set of conditionsincreasing as the Weight percent total carbon in the rings of thearomatic ring hydrocarbon compound content increases in the virgin oilfeed.

2. In a catalytic cracking process wherein a hydrocarbon feed selectedfrom the group consisting of fresh and virgin hydrocarbon iscontinuously cracked in the presence of catalystand the vaporous crackedproducts are 8 fractionated into gasoline, heating oil, cycle oil andbottoms, and cycle oil is to be recycled continuously to said crackingstep and cracked substantially to extinction, the improvement whichcomprises carrying out the cracking step at a relatively low totalconversion to coke, gasoline and lighter products below about 50% byweight calculated on a once-through basis, continuously fractionatingthe vaporous cracked products to recover a cycle oil fraction boilingbetween the 10 and 80% boiling points of the fresh oil feed,continuously recycling said cycle oil fraction to said cracking step andcracking it to extinction, the minimum amount of cycle oil fraction aspercent by volume on fresh feed oil beingdetermined according to theexpression where A is the weight percent of total carbon in the rings ofthe aromatic ring hydrocarbons in the fresh oil feed and B is the volumefraction of the fresh feed oil boiling in the range of the cycle oilbeing recycled.

3. A method according to claim 2 wherein A may vary between about 15 and30 and B may vary between about 0.3 and 0.7.

4. In a continuous catalytic cracking process wherein a hydrocarbon feedselected from the group consisting of fresh and virgin hydrocarbon oilhaving a boiling range of about 600l050 Ffis cracked in the presence ofa catalyst and the vaporous cracked products are continuouslyfractionated into gasoline, heating oil, cycle oil and bottoms, andcycle oil is to be continuously recycled to the cracking step, theimprovement which comprises carrying out the cracking step at arelatively low total conversion to coke, gasoline and lighter productsof about by weight calculated on a once-through basis, continuouslyfractionating the cracked products to recover a cycle oil fractionboiling between about 675 and 900 F. continuously recycling said cycleoil fraction to the cracking step and cracking it to extinction, theminimum amount of recycle oil as percent by volume on fresh feed oilbeing determined according to the expression and the maximum amount ofthe recycle oil as percent by volume on the fresh feed oil beingdetermined according to the expression where A is the weight percent oftotal carbon in the rings of the aromatic ring hydrocarbons in the freshoil feed and B is the volume fraction of the fresh feed oil boiling inthe range of the cycle oil being recycled and where A varies betweenabout 15 and 30 and B varies between about 0.3 and 0.7.

5. In a catalytic cracking process wherein virgin gas oil having aboiling range of about 600 F. to 1050 F. is cracked in the presence of asilica-alumina catalyst and vaporous cracked products are continuouslyfractionated into gasoline, heating oil, cycle oil and bottoms and cycleoil is continuously recycled to the cracking step, the improvement whichcomprises carrying out the cracking step at a relatively low totalconversion to coke, gasoline and lighter products at a w./hr./w. ofabout 12, continuously fractionating the cracked products to recover acycle oil fraction boiling between about 675 F. and 900 F., recycling atleast about by volume of said selected cycle oil fraction on the virgingas oil feed to the cracking step and cracking the selected cycle oilfraction to extinction.

(References on following page) References Cited in the file of thispatent UNITED STATES PATENTS 2,245,151 Pier June 10, 1941 2,332,794Hil-l Oct. 26, 1943 5 2,340,960 Hemminger Feb. 8, 1944 10 2,342,090Saohanen et a1. Feb. 15, 1944 2,436,257 Hansfor d et -al. Feb. 17, 1-948OTHER REFERENCES Brown at 211.: Ind. Eng. Chem. 38, 136 (1946). Brookset al.: The Chemistry of Petroleum Hydrocarbons, vol. 2, page 185, 186(1 955).

1. IN A CATALYSTIC CRACKING PROCESS WHEREIN VIRGIN HYDROCARBON OILHAVING A BOILING RANGE OF ABOUT 600*-1050*F. IS CRACKED IN THE PRESENCEOF A CATALYST AND THE VAPOROUS CRACKED PRODUCTS ARE CONTINUOUSLYFRACTIONATE INTO GASOLINE, HEATING OIL, CYCLE OIL AND BOTTOMS, AND CYCLEOIL IS TO BE CONTINUOUSLY RECYCLED TO THE CRACKING STEP AND THE SELECTEDCYCLE OIL FRACTION IS CRACKED TO EXTINCTION, THE IMPROVEMENT WHICHCOMPRISES CARRYING OUT THE CRACKING STEP AT A RELATIVELY LOW TOTALCONVERSION TO COKE, GASOLINE AND LIGHTER PRODUCTS BELOW ABOUT 49% BYWEIGHT CALCULATED ON A ONCE THROUGH BASIS, CONTINUOUSLY FRACTIONATINGTHE CRACKED PRODUCTS TO RECOVER A CYCLE OIL FRACTION BOILING BETWEENABOUT 675* AND 900*F., RECYCLING AT LEAST 50% BY VOLUME OF SAID SELECTEDCYCLE OIL FRACTION ON THE VIRGIN OIL FEED TO THE CRACKING STEP, THEAMOUNT OF RECYCLE OIL BEING RELATED TO THE WEIGHT PER CENT OF TOTALCARBON IN THE RINGS OF THE AROMATIC RING HYDROCARBON COMPOUND CONTENT OFTHE VIRGIN OIL FEED AND ALSO TO THE AMOUNT OF VIRGIN OIL FEEDCORRESPONDING IN BOILING RANGE TO THAT OF SAID SELECTED RECYCLE OILFRACTION,M THE AMOUNT OF RECYCLE OIL FOR A SELECTED SET OF CONDITIONSINCREASING AS THE WEIGHT PERCENT TOTAL CARBON IN THE RINGS OF THEAROMATIC RING HYDROCARBON COMPOUND CONTENT INCREASES IN THE VIRGIN OILFEED.